DMA : Computer Hardware Buyers’ Glossary

DMA (Direct Memory Access).
PCs (Personal Computers)
have a pair of tiny computers in addition to the main CPU (Central Processing Unit).
They are quite stupid. All they can do is arrange to send data from
RAM (Random Access Memory)
to an I/O device or vice versa. The advantage is, they can do this without requiring
much help from the main CPU. This frees the main CPU
to do more
useful work. Most of the time you can forget about DMA.
However, if
you install a sound card, you may have to assign the card some DMA
channels that don’t conflict with any other device in your machine. In
theory, Plug & Play sound cards will automatically select unused
DMA cards, but
does not always work. Each device that wants to use DMA
needs its own
private channel. Each controller can keep four conversations going at once. Each conversation gets assiged to a
channel number. Here is how the DMA
channels are
assigned:

The ordinary slow style DMA
is called 3rd party DMA because the DMA
controller acts as a third party to mediate between two other devices (usually
RAM and a
peripheral). Because these built-in DMA
controllers are so slow and so ineptly designed, sometimes a
PCI (Peripheral Component Interconnect)
card or ISA (Industry Standard Architecture) SCSI (Small Computer System Interface) card will
provide its own replacement high speed DMA
controller that runs at full bus speed. This is called first party
DMA . The
standard DMA controller is so incredibly slow that,
PIO (Programmed Input/Output) is faster for hard disk access.